Separable pedicle screw

ABSTRACT

Techniques are generally described related to a method and system for treating an injury with a bone screw. One example bone screw may be configured to fracture at a pre-selected frangible location so that the point of failure is not in an inaccessible location, e.g., deeply embedded below the surface of a treated bone. The bone screw may further include a material disposed over the frangible location that is designed to temporarily strengthen the screw and selected to be absorbed by the body over a period of time after installation of the screw in the bone. One example bone screw may include an unthreaded portion that is configured to facilitate removal of an embedded screw fragment from a bone in the event that the screw fails in vivo.

REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Phase entry under 35 U.S.C. §371of International Application PCT/CN2010/075630, filed Aug. 2, 2010,entitled “SEPARABLE PEDICLE SCREW,” the entirety of which isincorporated herein by reference.

BACKGROUND

Unless otherwise indicated herein, the approaches described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

Bone screws have been used in spinal instrumentation since the 1960s tomanage bone fractures and correct deformity. For example, the pediclescrew may provide a means of gripping a spinal segment, where the screwacts as a firm anchor point in one vertebra that can be connected toother such anchor points in other vertebrae with a rod. With two or moreconsecutive vertebrae fixated by such a construct, motion between thevertebrae is prevented, which assists in spinal fusion procedures byholding bony structures together. Many surgeons also believe thatpedicle screws enhance patient recovery because they provide immediatestability for the spine and early mobilization for the patient.

SUMMARY

In accordance with one embodiment of the disclosure, a bone screwincludes a shank, a threaded portion formed on an end of the shank, afrangible region formed in the shank, and an unthreaded portion disposedbetween the threaded portion and the frangible portion, wherein theunthreaded portion is configured to facilitate removal of the threadedportion of the screw from a bone when the frangible portion of the screwhas broken after installation of the screw in the bone.

In accordance with another embodiment of the disclosure, a pedicle screwincludes a shank, a threaded portion formed on an end of the shank, andan unthreaded portion disposed in a center portion of the shank, whereinthe unthreaded portion is configured to facilitate removal of thethreaded portion of the screw from a bone.

In accordance with a further embodiment of the disclosure, a method fortreating an injury includes selecting a bone screw having a threadedportion formed on an end of a shank, a frangible region formed in theshank, and an unthreaded portion disposed between the threaded portionand the frangible portion, wherein the unthreaded portion is configuredto facilitate removal of the threaded portion of the screw from the bonewhen the frangible portion of the screw has broken after installation ofthe screw in the bone, and installing the screw into a bone.

The foregoing summary is illustrative only and is not intended to be inany way limiting. In addition to the illustrative aspects, embodiments,and features described above, further aspects, embodiments, and featureswill become apparent by reference to the drawings and the followingdetailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of an illustrative embodiment of a bonescrew;

FIGS. 2A-E show cross-sectional views of illustrative embodiments of thebone screw in FIG. 1, taken from section B-B;

FIG. 3 shows a cross-sectional view of an illustrative embodiment of thebone screw in FIG. 1, taken from section A-A, in which a frangibleportion is a reduced-cross-sectional portion of the shank of the bonescrew;

FIGS. 4A-D show expanded schematic views of illustrative embodiments ofthe frangible region indicated in FIG. 1, where the frangible region isan annular ring;

FIG. 5 shows an expanded schematic view of an illustrative embodiment ofthe frangible region indicated in FIG. 1, in which an absorbablematerial is disposed over the frangible region;

FIG. 6 shows a schematic view of an illustrative embodiment of a bonescrew installed in a bony structure;

FIG. 7 shows a schematic view of an illustrative embodiment of a bonescrew installed in a bony structure after a frangible region of the bonescrew has broken;

FIG. 8 shows a schematic view of an illustrative embodiment of a bonescrew being removed from a bony structure after mechanical failure ofthe bone screw at a frangible region.

FIG. 9 shows a schematic view of an illustrative embodiment of a bonescrew installed in a bony structure, in which an unthreaded portion ofthe screw is partially embedded in the bony structure; and

FIG. 10 shows a flow chart of an illustrative embodiment of a method oftreating an injury.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. In the drawings,similar symbols typically identify similar components, unless contextdictates otherwise. The illustrative embodiments described in thedetailed description, drawings, and claims are not meant to be limiting.Other embodiments may be utilized, and other changes may be made,without departing from the spirit or scope of the subject matterpresented here. It will be readily understood that the aspects of thedisclosure, as generally described herein, and illustrated in theFigures, can be arranged, substituted, combined, and designed in a widevariety of different configurations, all of which are explicitlycontemplated and make part of this disclosure.

While bone screws have long been used for a large number ofapplications, some drawbacks are known. For example, bone screws aretypically left in place permanently to avoid the complications that mayarise from reoperative surgery. Over time, such screws may fracture andfail, requiring removal. Because the point of failure of a bone screw iscommonly in a threaded portion of the screw or at a junction between athreaded portion and an unthreaded portion of the screw, the brokenscrew fragment, i.e., the screw tip, can be deeply embedded in thenow-healed bone material. Removal of such a screw fragment can involveenlarging the bony tunnel around the screw fragment to provide access tothe fragment, possibly causing excessive loss of bone mass and/or bonefracture. Alternatively, the screw fragment may have to remain in thebone, which can be physiologically and psychologically problematic forthe patient.

Embodiments disclosed herein contemplate a bone screw configured tosimplify the removal of the screw in the event that the screw fails invivo. According to some embodiments, a bone screw may be configured tofracture at a pre-selected frangible location so that the point offailure is not in an inaccessible location, e.g., deeply embedded belowthe surface of a treated bone. The bone screw may further include amaterial disposed over the frangible location that is designed totemporarily strengthen the screw and selected to be absorbed by the bodyover a period of time after installation of the screw in the bone.According to some embodiments, a bone screw may include an unthreadedportion that is configured to facilitate removal of an embedded screwfragment from a bone in the event that the screw fails in vivo.

FIG. 1 shows a schematic diagram of an illustrative embodiment of a bonescrew 100. Bone screw 100 may include a threaded portion 101 formed on atip thereof, an unthreaded portion 102, a frangible portion 103, and ahead 104. Head 104 may be a typical bone screw head. For example, whenbone screw 100 is a pedicle screw used to anchor two or more consecutivespine segments together, head 104 may include a clamp fixture 109 formechanically coupling a connecting rod to bone screw 100. Bone screw 100may be constructed of stainless steel or other durable surgical-gradematerials, such as titanium and titanium-containing alloys.

Threaded portion 101 may be formed into the shank of bone screw 100 andmay be substantially similar in configuration to the threaded portion oftraditional pedicle screws. To wit, in some embodiments, threadedportion 101 may be self-tapping to simplify installation and/or may havea thread depth that increases toward the tip to enhance pulloutstrength. A length 110 and major diameter 111 of threaded portion 101may vary depending on the size of bone into which bone screw 100 will beinstalled and the desired depth of placement of bone screw 100. Forexample, length 110 and major diameter 111 may be smaller forinstallation in smaller bones, such as the thoracic vertebrae, than forinstallation in larger bones, such as the lumbar and sacral vertebrae.

Unthreaded portion 102 may be formed into the shank of bone screw 100adjacent to threaded portion 101 and is configured to facilitate removalof threaded portion 101 from a bone if bone screw 100 fails in vivoafter installation. For example, in some embodiments, unthreaded portion102 may include at least one pair of flat and substantially parallelsides, thereby providing gripping surfaces for unscrewing threadedportion 101 from a bone when threaded portion 101 is completely embeddedin a bone. In some embodiments, as shown in FIG. 1, unthreaded portion102 may have a cross section having a polygonal shape, e.g., square,triangular, hexagonal, etc., so that a sleeved tool configured to fitthe cross-section of the unthreaded portion 102 can be used to unscrewthreaded portion 101 from a bone when threaded portion 101 is completelyembedded in a bone.

FIGS. 2A-E show cross-sectional views of illustrative embodiments ofbone screw 100, taken from section B-B in FIG. 1. In FIG. 2A, unthreadedportion 102 is hexagonal in cross section and has a width 201 that issubstantially wider than major diameter 111. Such a hexagonal crosssection facilitates the use of an inner-hexagon sleeved tool to unscrewthreaded portion 101 after bone screw 100 has failed and head 104 hasseparated from unthreaded portion 102. Unthreaded portion 102 can beused to greatly simplify the removal of threaded portion 101 even whenthreaded portion 101 is completely embedded in a bone, since such aprocedure does not require removal of bone material surrounding threadedportion 101. In addition, because a width 201 of the hexagonal crosssection of unthreaded portion 102 is wider than major diameter 111,unthreaded portion 102 is configured to mechanically interfere with thecircular opening formed in a bone by threaded portion 101, therebyacting as a mechanical stop that prevents unthreaded portion 102 frombeing deeply embedded in a bone during installation of bone screw 100.In such an embodiment, unthreaded portion 102 can be readily accessedwithout the need for removing a significant amount of bone material.

In FIGS. 2B-2D, the cross section of unthreaded portion 102 isillustrated as having other geometric shapes, specifically a square, acircle having two substantially parallel flats, and a triangle,respectively. In such embodiments, the cross-sectional shape ofunthreaded portion 102 may be mated to a sleeved tool configured with anappropriately shaped inner-sleeve. Other geometrical shapes for thecross-section of unthreaded portion 102 are also contemplated, such asstar drives or other shapes that can be mated to a corresponding sleevedtool. As shown in FIGS. 2B-2D, the cross section of unthreaded portion102 may be configured to interfere with a circular opening in a boneformed by having a diameter equal to the major diameter of the threadedportion, thereby acting as a mechanical stop that prevents unthreadedportion 102 from being deeply embedded in a bone during installation ofbone screw 100.

In other embodiments, unthreaded portion 102 may be configured to have awidth or diameter that is substantially equal to major diameter 111, butlarger than a minor diameter 112 of threaded portion 101, so thatunthreaded portion 102 does not act as a mechanical stop against a boneduring installation of bone screw 100. FIG. 2E illustrates one suchembodiment. Because unthreaded portion 102 does not act as a mechanicalstop against a bone during installation of bone screw 100, unthreadedportion 102 may be embedded partially in the bone.

Referring back to FIG. 1, frangible portion 103 may be formed in theshank of bone screw 100 so that if, over time, bone screw 100 fails, thepoint of failure may be at frangible portion 103, rather that at aportion of bone screw 100 that is embedded in bone and relativelyinaccessible. Frangible portion 103 may be disposed between head 104 andunthreaded portion 102, so that if bone screw 100 fails, threadedportion 101 and unthreaded portion 102 will make up one fragment of bonescrew 100. Consequently, such a fragment can be readily removed by usingunthreaded portion 102 to unscrew the fragment from the bone. In someembodiments, frangible portion 103 is a reduced-cross-section portion ofthe shank of bone screw 100. FIG. 3 shows a cross-sectional view of anillustrative embodiment of bone screw 100, taken from section A-A, inwhich frangible portion 103 is a reduced-cross-section portion of theshank of bone screw 100. In FIG. 3, the unhatched area 302 indicates thecross-section of frangible portion 103 and hatched areas 301 indicatethe difference in cross-sectional area between unthreaded portion 102and frangible portion 103, for example where material may have beenremoved from the shank of bone screw 100 to form unhatched area 302 andfrangible portion 103. In such an embodiment, frangible portion 103 mayhave a lower cross-sectional area than the cross-sectional area ofunthreaded portion 102 and/or the cross-sectional area of threadedportion 101. In some embodiments, frangible portion 103 may have across-sectional area that is approximately 90% or less than thecross-sectional area of unthreaded portion 102. In some embodiments,frangible portion 103 may have a cross-sectional area that is less thanthe area of a circle having a diameter equal to major diameter 111. Insome embodiments, frangible portion 103 may have a cross-sectional areathat is less than the area of a circle having a diameter equal to minordiameter 112. In such embodiments, the reduced cross-sectional area offrangible region 103 greatly increases the likelihood of bone screw 100failing at frangible region 103 rather than at an inaccessible region ofthreaded portion 101. In addition, the fact that frangible region 103 ispositioned closer to the midpoint of bone screw 100 than threadedportion 101 is to the midpoint of bone screw 100 further increases thelikelihood that any mechanical failure of bone screw 100 will take placeat frangible region 103. The hatched regions in FIG. 3 illustrate onlyone example of how frangible region 103 may have areduced-cross-sectional area with respect to unthreaded portion 102and/or threaded portion 101. In some embodiments, frangible region 103may be an annular ring formed in unthreaded portion 102. FIGS. 4A-D showexpanded schematic views of illustrative embodiments of frangible region103 indicated in FIG. 1, where frangible region 103 is an annular ring.In FIG. 4A, the annular ring that makes up frangible region 103 has asemi-circular profile. For reference, major diameter 111 of threadedportion 101 is included in FIG. 4A. As shown, the diameter for frangibleregion 103 may be significantly less than major diameter 111 to increasethe likelihood that the failure point of bone screw 100 will befrangible region 103, rather than a region of threaded portion 101. InFIG. 4B, the annular ring that makes up frangible region 103 has arectangular profile. In FIGS. 4C and 4D, the annular ring that makes upfrangible region 103 has a profile that includes a triangular in-cut 401that may act as a scribe mark. Since the mode of failure of bone screw100 is typically a fatigue failure, in-cut 401 may further increase thelikelihood that any failure of bone screw 100 will take place atfrangible region 103.

According to some embodiments, bone screw 100 may include a materialdesigned to temporarily strengthen bone screw 100, where the material isselected to be absorbed by the body over a period of time afterinstallation of the screw in the bone. The absorbable material may bedisposed over frangible region 103 to structurally support the portionof bone screw 100 that is weakened by the presence of frangible region103. In such embodiments, the absorbable material reinforces bone screw100 during the time that bony structures anchored together by one ormore bone screws 100 fuse together and increase in structural strength.As the treated bony structures become stronger, bone screw 100 isslightly weakened in frangible region 103 because the absorbablematerial is slowly absorbed by the body of the patient. Consequently,once the treated bony structures have healed, frangible region 103 maybe slightly weakened so that the failure point of bone screw 100 will bein the desired location. The one or more absorbable materials disposedover frangible region 103 may be selected based on mechanicalproperties, i.e., modulus of elasticity, etc., and absorbability by thehuman body. Example absorbable materials include calcium phosphate andcalcium sulphate.

FIG. 5 shows an expanded schematic view of an illustrative embodiment offrangible region 103 indicated in FIG. 1, in which an absorbablematerial 501 is disposed over frangible region 103. In some embodiments,absorbable material 501 is disposed over frangible region 103 so that asurface 502 is formed that is substantially flush with adjacent surface503 of unthreaded portion 102. In some embodiments, the depth 504 of afeature forming frangible region 103 may be selected based on theabsorbability of absorbable material 501 by the human body.Specifically, for a relatively fast-absorbing material, frangible region103 may be configured with a depth 504 so that absorbable material 501is not completely absorbed for six to eight months after installation ofbone screw 100. Thus, the absorption time of absorbable material 501 maybe determined for bone screw 100 by the configuration of frangibleregion 103. Alternatively, in other embodiments absorbable material 501may be selected based on the geometry of frangible region 103. Thus, theabsorption time of absorbable material 501 may be determined for bonescrew 100 by the specific absorbable material 501 selected for bonescrew 100.

In operation, bone screw 100 may be inserted into a bony structure, suchas a pedicle bone of a thoracic, sacral, or lumbar vertebra, to act asan anchor for a connecting rod and/or to hold one or more other bonystructures in place as an adjunct to spinal fusion surgery. FIG. 6 showsa schematic view of an illustrative embodiment of a bone screw 100installed in a bony structure 601. In order to determine the desireddepth and angle for placement of bone screw 100, a surgeon may usefluoroscopy and/or conventional x-ray analysis. A receiving channel isthen typically drilled in the desired location and at the desired angleand bone screw 100 is inserted into the receiving channel. In theembodiment illustrated in FIG. 6, bone screw 100 is inserted into bonystructure 601 until the mechanical stop formed by unthreaded portion 102contacts surface 604 of bony structure 601. In other embodiments,unthreaded portion 102 may not form such a mechanical stop, and bonescrew 100 may be inserted until unthreaded portion 102 is partiallyembedded in bony structure 601. In some embodiments, a rod 602 may beattached to bone screw 100 by a connecting screw 603. Rod 602 can thenbe used to mechanically couple bony structure 601 to an adjacent anchor,such as another bone screw (not shown) configured with a clamp fixture.

FIG. 7 shows a schematic view of an illustrative embodiment of bonescrew 100 installed in bony structure 601 after frangible region 103 hasmechanically failed. In FIG. 7, bony structure 601 and other bonystructures (not shown) treated by bone screw 100 have healed andincreased in mechanical strength, but over time, e.g., after about 6 to12 months, bone screw 100 has undergone fatigue failure. It is notedthat the point of failure 701 of bone screw 100 coincides with frangibleregion 103. Because bony structure 601 has healed, bone screw 100 is nolonger required and may be removed rather than replaced. FIG. 8 shows aschematic view of an illustrative embodiment of bone screw 100 beingremoved from bony structure 601 after mechanical failure of bone screw100 at frangible region 103. Because bone screw 100 has broken atfrangible region 103, unthreaded portion 102 is exposed and may be usedto unscrew the screw fragment, i.e., unthreaded portion 102 and threadedportion 101, remaining in bony structure 601. A sleeved tool 605 is usedto remove the screw fragment, where sleeved tool 605 is configured tomatch the cross section, e.g., hexagon, square, triangle, etc. ofunthreaded portion 102. FIG. 9 shows a schematic view of an illustrativeembodiment of a bone screw 610 installed in bony structure 601, in whichunthreaded portion 102 of bone screw 100 is partially embedded in bonystructure 601. In such an embodiment, the width of unthreaded portion102 is substantially equal to major diameter 111 of threaded portion101, i.e., only slightly larger or smaller than major diameter 111, andtherefore can be substantially embedded in bony structure 601. In orderto remove threaded portion 101 and unthreaded portion 102 aftermechanical failure of bone screw 610 and the removal of head 104, asmall amount of bony structure 601 may be removed to improve access tounthreaded portion 102. For purposes of this particular exemplaryillustration, the reduced cross-section of frangible area 103 isexaggerated in FIGS. 6-9. In some embodiments, the total cross-sectionalarea of frangible region 103 may only be about 10% to 15% less than thecross-sectional area of unthreaded portion 102.

FIG. 10 shows a flow chart of an illustrative embodiment of a method1000 of treating an injury. For ease of description, method 1000 isdescribed in terms of a bone screw substantially similar to bone screw100 in FIG. 1. However, other configurations of bone screw may alsoperform method 1000. Method 1000 may include one or more operations,functions, or actions as illustrated by blocks 1001, 1002, and/or 1003.The various blocks are not intended to be limiting to the describedembodiments. For example, one skilled in the art will appreciate that,for this and other processes and methods disclosed herein, the functionsperformed in the processes and methods may be implemented in differingorder. Furthermore, the outlined steps and operations are only providedas examples, and some of the steps and operations may be optional,combined into fewer steps and operations, or expanded into additionalsteps and operations without detracting from the essence of thedisclosed embodiments. Prior to block 1001, traditional techniques, suchas fluoroscopy or conventional x-ray analysis, may be performed todetermine the depth and angle for screw placement, and an appropriatereceiving channel, such as a pilot hole, may be formed in the treatedbone.

In block 1001 (SELECT BONE SCREW), a bone screw 100 is selected havingthreaded portion 101 formed on an end of a shank, frangible region 103formed in the shank, and unthreaded portion 102 disposed betweenthreaded portion 101 and frangible portion 103. Unthreaded portion 102may be configured to facilitate removal of threaded portion 101 from abone when frangible portion 103 has broken after installation of bonescrew 100 in a bone. For example, unthreaded portion 102 may have ahexagonal cross section configured to mate with the inner sleeve of asleeved tool. In some embodiments, the selection of bone screw 100 maybe based on a desired depth of insertion for bone screw 100 into thebone and a length of threaded portion 101.

In block 1002 (INSTALL BONE SCREW), bone screw 100 is inserted in thebone until threaded portion 101 is substantially embedded in the boneand at least a portion of unthreaded portion 102 is exposed outside thebone. In some embodiments, bone screw 100 may be inserted in the bonevia a pilot hole. In some embodiments, unthreaded portion 102 may beconfigured to interfere with the circular opening formed in the bone bythreaded portion 101, thereby acting as a mechanical stop. In suchembodiments, unthreaded portion 102 may be wider than major diameter 111of threaded portion 101.

In block 1003 (REMOVE SCREW FRAGMENT), a screw fragment is removed fromthe bone after bone screw 100 has undergone mechanical failure atfrangible portion 103. In some embodiments, the screw fragment isremoved from the bone using unthreaded portion 102. In some embodiments,a sleeved tool configured to fit the cross-section of unthreaded portion102 may be used to unscrew the screw fragment from the bone. In someembodiments, a small amount of bone material may be removed aroundunthreaded portion 102 to provide more access thereto for the sleevedtool.

The foregoing detailed description has set forth various embodiments ofthe devices and/or processes via the use of block diagrams, flowcharts,and/or examples. Insofar as such block diagrams, flowcharts, and/orexamples contain one or more functions and/or operations, it will beunderstood by those within the art that each function and/or operationwithin such block diagrams, flowcharts, or examples can be implemented,individually and/or collectively, by a wide range of hardware, software,firmware, or virtually any combination thereof.

The herein described subject matter sometimes illustrates differentcomponents contained within, or connected with, different othercomponents. It is to be understood that such depicted architectures aremerely exemplary, and that in fact many other architectures can beimplemented which achieve the same functionality. In a conceptual sense,any arrangement of components to achieve the same functionality iseffectively “associated” such that the desired functionality isachieved. Hence, any two components herein combined to achieve aparticular functionality can be seen as “associated with” each othersuch that the desired functionality is achieved, irrespective ofarchitectures or intermedial components. Likewise, any two components soassociated can also be viewed as being “operably connected”, or“operably coupled”, to each other to achieve the desired functionality,and any two components capable of being so associated can also be viewedas being “operably couplable”, to each other to achieve the desiredfunctionality. Specific examples of operably couplable include but arenot limited to physically mateable and/or physically interactingcomponents.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations may be expressly set forth herein for sakeof clarity.

It will be understood by those within the art that, in general, termsused herein, and especially in the appended claims (e.g., bodies of theappended claims) are generally intended as “open” terms (e.g., the term“including” should be interpreted as “including but not limited to,” theterm “having” should be interpreted as “having at least,” the term“includes” should be interpreted as “includes but is not limited to,”etc.). It will be further understood by those within the art that if aspecific number of an introduced claim recitation is intended, such anintent will be explicitly recited in the claim, and in the absence ofsuch recitation no such intent is present. For example, as an aid tounderstanding, the following appended claims may contain usage of theintroductory phrases “at least one” and “one or more” to introduce claimrecitations. However, the use of such phrases should not be construed toimply that the introduction of a claim recitation by the indefinitearticles “a” or “an” limits any particular claim containing suchintroduced claim recitation to inventions containing only one suchrecitation, even when the same claim includes the introductory phrases“one or more” or “at least one” and indefinite articles such as “a” or“an” (e.g., “a” and/or “an” should typically be interpreted to mean “atleast one” or “one or more”); the same holds true for the use ofdefinite articles used to introduce claim recitations. In addition, evenif a specific number of an introduced claim recitation is explicitlyrecited, those skilled in the art will recognize that such recitationshould typically be interpreted to mean at least the recited number(e.g., the bare recitation of “two recitations,” without othermodifiers, typically means at least two recitations, or two or morerecitations). Furthermore, in those instances where a conventionanalogous to “at least one of A, B, and C, etc.” is used, in generalsuch a construction is intended in the sense one having skill in the artwould understand the convention (e.g., “a system having at least one ofA, B, and C” would include but not be limited to systems that have Aalone, B alone, C alone, A and B together, A and C together, B and Ctogether, and/or A, B, and C together, etc.). In those instances where aconvention analogous to “at least one of A, B, or C, etc.” is used, ingeneral such a construction is intended in the sense one having skill inthe art would understand the convention (e.g., “a system having at leastone of A, B, or C” would include but not be limited to systems that haveA alone, B alone, C alone, A and B together, A and C together, B and Ctogether, and/or A, B, and C together, etc.). It will be furtherunderstood by those within the art that virtually any disjunctive wordand/or phrase presenting two or more alternative terms, whether in thedescription, claims, or drawings, should be understood to contemplatethe possibilities of including one of the terms, either of the terms, orboth terms. For example, the phrase “A or B” will be understood toinclude the possibilities of “A” or “B” or “A and B.”

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

We claim:
 1. A bone screw comprising: a shank; a threaded portion formedon an end of the shank; a frangible region formed in the shank; amaterial disposed over the frangible region, wherein the material is anabsorbable material selected to be absorbable in-vivo after installationof the screw in the bone; and an unthreaded portion disposed between thethreaded portion and the frangible region, wherein the unthreadedportion is configured to facilitate removal of the threaded portion ofthe screw from a bone when the frangible region of the screw has brokenafter installation of the screw in the bone; wherein the frangibleregion is a reduced-cross-section portion of the shank; wherein thereduced-cross-section portion is an annular groove; wherein the materialis selected from the group of absorbable materials consisting of calciumphosphate and calcium sulphate; wherein the unthreaded portion comprisesat least one pair of flat and substantially parallel sides.
 2. The bonescrew of claim 1, wherein the unthreaded portion is hexagonal in crosssection.
 3. The bone screw of claim 1, wherein a cross section of theunthreaded portion is configured to interfere with a circular openinghaving a diameter equal to a major diameter of the threaded portion. 4.The bone screw of claim 1, wherein a diameter of the unthreaded portionis less than a minor diameter of the threaded portion.
 5. The bone screwof claim 1, wherein the material disposed over the frangible regionextends over the frangible region to form a surface flush with a surfaceof the unthreaded portion.
 6. A pedicle screw comprising: a shank; afrangible region formed in the shank, wherein the frangible region is areduced-cross-section portion of the shank; an absorbable materialdisposed over the frangible region, wherein the absorbable material isselected to be absorbable in-vivo after installation of the screw in thebone; a threaded portion formed on an end of the shank; and anunthreaded portion disposed in a center portion of the shank, whereinthe unthreaded portion is configured to facilitate removal of thethreaded portion of the screw from a bone.